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Glacier Lesson 6: The Story of a Lonely Rock

This huge rock is in the Willamette Valley in Oregon. It is different from all the other rocks around it.

How did it get there? Where did it come from?

Here is one clue: it has parallel scratch marks on it! Those scratch marks are found on rocks that have been dragged by glaciers. (We wrote about that right here.)

So we are very sure that this rock was scratched when it was dragged by a glacier.

What else do we know about this rock?

Scientists have taken pieces of this rock and analyzed the minerals. The minerals are the same as rocks from northwest Montana. That’s over 500 miles away!

So we have good evidence that this rock came from northwest Montana and that a glacier dragged it around.

Were there glaciers in northwest Montana?

Yes, at the end of the last ice age, northwestern Montana was partially covered by glaciers. But this rock is in the Willamette Valley of Oregon! The glacier was not 500 miles long.

But there is a way that a piece of a glacier could have gotten to Oregon. It could have floated there, as an iceberg!

Floated on what?

There is no ocean between Montana and Oregon. There is a river though. Well, not one river, but a system of rivers, which flow from Montana all the way to the ocean. Those rivers flow into the Columbia River, and this rock is near the Willamette River which flows into the Columbia.

Could an iceberg from a Montana glacier have made it all the way to Oregon?

Yes!

There is a lot off evidence that there was a huge lake in northwestern Montana at the end of the last ice age. A huge glacier from Canada made an ice dam on the Clark Fork River. The lake that formed behind the dam is called Glacial Lake Missoula. It was more than a thousand feet deep at the dam, and it was huge. Glacial Lake Missoula covered almost 3,000 square miles, and held more water than Lake Erie and Lake Ontario combined!

(Actually, the glacier dammed the river several times. But the first lake that formed seems to have been the deepest and the biggest, and we are going to talk about that one.)

When that first ice dam broke, all that water behind it burst out in a huge wall. In some places, the wall of water was more than 500 feet high!! There is evidence that the water moved through the narrow valleys at almost 60 miles an hour!

We can tell how deep the water was, because it stripped off the soil of the valley walls, stripped it right down to the rock. Multiply the height of the valley by the width and multiply that by the speed of the flow and you get the volume of flow per hour: 9 cubic MILES of water per hour.

Let’s try to imagine 9 cubic miles of water. The next time you go outside, look to the east, where the sun comes up. Look at something you think is two miles away, and imagine a line going there. Now look to your right, to the south. Look at something in that direction that is two miles away. Imagine that line. Those two lines make a square two miles on a side. It has an area of 2 x 2 = 4 square miles.

Now look straight up and try to imagine a line two miles high. You are now standing at the corner of an imaginary cube, 2 miles on each side. Your cube has a volume of 2 x 2 x 2 = 8 cubic miles.

Now fill that cube with water.

Make the sides just a bit longer, to just under two and a tenth miles (2.08, actually), and you have 9 cubic miles of water.

That’s how much water flowed at the peak of the flood from Glacial Lake Missoula. In an hour!

How does that compare to other rivers?

The Amazon has the largest flow of any river in the world: about one hundredth of a cubic mile per hour. So the flow of the Glacial Lake Missoula Flood was 900 times bigger!

Try to imagine 900 Amazons in one flood!

A cube of water one yard on each side weighs just about 1,700 pounds, just under a ton. Imagine the weight of YOUR cube of water, two MILES on each side. Imagine the force and the power of that water as it raged through the canyons at 60 miles an hour, with giant whirlpools breaking and stripping the rock itself. Imagine the roar of the water, the crashing of great boulders slamming into each other and into the valley walls.

That would make a great movie!

But let’s get back to our rock.

The rock has parallel scratch marks. That tells us it was once in a glacier. The minerals of the rock tell us it came from northwest Montana. Other evidence tells us there was once a huge lake in behind an ice dam in northern Montana. That lake had glaciers on its shores, which means there were icebergs on the lake.

When the ice dam broke, it unleashed a gigantic, humongous, terrifying, titanic, stupendous, colossal, awesome monster of a flood, which ripped apart and re-arranged the land as it raged to the sea, carrying icebergs which carried rocks and carried this rock 500 miles.

What a trip this rock has taken, what a tale this rock could tell!

Oh wait. It IS telling us.

All we have to do is pay attention, collect the facts, and imagine the story that explains the facts. When we do that, this rock tells us a great story: of the humungous floods from Glacial Lake Missoula.

You want to learn more about the story of Glacial Lake Missoula and the Humungous Flood? You could start here, and explore the links.